There are some known technologies that are related to an optical rotary encoder. For example, a technology is known by which a rotation speed and rotation angle of a detection target in which a member to be detected is installed is highly accurately detected. In such a technology, first, three rotation sensors that are arranged at 90° intervals in the circumferential direction of a disc-like code wheel attached to an output shaft of a motor individually output signals that correspond to a rotation speed of the code wheel. An error component of one cycle (one-cycle component) occurs per rotation when the code wheel is eccentric to the output shaft, and an error component of two cycles (two-cycle component) occurs per rotation when the code wheel is deformed into an oval shape. Here, subtraction or addition is performed on a correction signal that is obtained by removing the two-cycle component by averaging outputs of a first rotation sensor and a second rotation sensor and a correction signal that is obtained by removing the two-cycle component by obtaining a difference between outputs of the first rotation sensor and a third rotation sensor, in accordance with a phase and amplitude of the remaining one-cycle component. As described above, in such a technology, both of the error component of one-cycle per rotation and the error component of two-cycle per rotation are removed by one rotation of the member to be detected (see, for example, Japanese Laid-open Patent Publication No. 2005-168280).
In addition, a technology of a rotary encoder is known by which an eccentricity amount may be appropriately corrected even if eccentricity of a disk varies for each rotation. In such a technology, the rotary encoder includes a rotation disk, a main sensor, and an auxiliary sensor. Here, the rotation disk has a main slit pattern that is used to detect a rotation angle of the rotation disk and a plurality of concentric auxiliary slit patterns that are used to detect eccentricity of the rotation disk. The main sensor reads the main slit pattern. In addition, the auxiliary sensor is arranged at a position of 90° with respect to the main sensor and detects the auxiliary slit pattern. In such a technology, an angle detection signal of the main sensor is corrected using a signal from the auxiliary sensor (see, for example, Japanese Laid-open Patent Publication No. 2001-264119).
In addition, a technology of an optical encoder is known by which a measurement error due to eccentricity of a pulse plate is minimized with a configuration at low cost. In such a technology, a slit pattern of the pulse plate is formed so as to generate an angle (for example, 45°) with respect to the traveling direction of the pulse plate (see, for example, Japanese Laid-open Patent Publication No. 2004-109074).
In addition, a technology is known by which eccentricity of an encoder scale member of a rotation encoder is determined. In such a technology, first, an encoder scale material having the geometric center is prepared. Next, the center of the encoder scale material is located at a certain position. Next, the encoder scale member is formed by creating a scale in the encoder scale material. As a result, the center of the scale of the encoder scale member is located at the certain position. In such a technology, here, eccentricity between the above-described geometric center and the above-described certain position is measured (see, for example, Japanese National Publication of International Patent Application No. 2008-539407).